Pulsed power supplies are generally known. They are used for matching electrical energy provided to the requirements of a load. This is, for example, matching of the voltage level or provision of a controlled voltage or current source. Pulsed power supplies essentially comprise at least one electronic switch and energy stores such as inductors and capacitors.
In common use is the so-called buck converter, which is used, in a representative manner for other converters, as the basis for describing the present invention. Buck converters are usually used for providing a voltage source which has a voltage which is lower than the voltage of the feed source. Buck converters, however, are also used to operate a load from a voltage source, which load requires a constant current or a constant power. This is the case with discharge lamps. In particular, high-pressure discharge lamps are usually fed by a buck converter.
The buck converter has an electronic switch which is switched on or closed for the duration of an On time and is switched off or open for the duration of an Off time. During the On time, energy flows from the feed source to a buck inductor. During the Off time, the energy stored in the buck inductor flows to the load. The ratio between the On time and the Off time defines the so-called duty cycle, by means of which the energy flow and thus the load voltage or the load current can be controlled. It is therefore necessary for a circuit arrangement to be provided for controlling the pulsed power supply, in this case the buck converter, which produces a control signal for the purpose of switching the electronic switch on, the duty cycle being dependent on a controlled variable.
A typical requirement of such a control circuit is a control speed which is as high as possible in order that changes in load cannot be perceived in the controlled output variable. A control speed which is as high as possible is also necessary if the controlled output variable is intended to follow a desired temporal profile in a manner which is as fault-free as possible. This is particularly the case for the current profile of high-pressure discharge lamps in projection applications. With this application, it is also necessary for control oscillations to be as low as possible.
A circuit arrangement for controlling a pulsed power supply is known from the prior art which functions in the so-called “average current mode”. The control signal for the electronic switch is in this case produced by a comparator which compares a saw-tooth voltage of a saw-tooth generator with a threshold value which is dependent on the controlled variable. The threshold value is provided by an integrating differential amplifier, which detects the current through the buck inductor. A desired current value can be input as a reference value to the differential amplifier. The pulsed power supply then acts as a current source, for example for a discharge lamp. The output voltage of the pulsed power supply can also be input, as the reference value, to the differential amplifier. In this case, the pulsed power supply functions as a voltage source.
The implementation of the circuit arrangement for controlling the pulsed power supply in accordance with the prior art described has the following disadvantages:                The control speed cannot be increased as desired, since otherwise the stability of the control is no longer ensured. An increase in the control speed, for example by increasing the so-called closed-loop gain, is always detrimental to stability in the prior art and thus leads to increased control oscillations.        When the threshold value is reached, the electronic switch does not operate immediately but only after a certain delay, owing to unavoidable response times. The change in the buck inductor current during this delay is dependent on the gradient of the change in the buck inductor current. This in turn is dependent on the voltage of the energy feed source. With different voltages of the energy feed source, different peak values result for the buck inductor current. In the prior art, variations in the voltage of the energy feed source thus in principle lead to variations in the controlled variable, i.e. variations in the voltage of the energy feed source cannot be completely compensated for. Better compensation can in turn only take place to the detriment of stability.        